1. Field of the Invention
The present invention relates to an enhanced free layer for a spin valve sensor and, more particularly, to such a free layer and a method of making wherein a desirable negative ferromagnetic coupling field is maintained when a copper layer is located between the free layer and a capping layer for the purpose of increasing a magnetoresistive coefficient dr/R of the spin valve sensor.
2. Description of the Related Art
The heart of a computer is a magnetic disk drive which includes a rotating magnetic disk, a slider that has read and write heads, a suspension arm above the rotating disk and an actuator arm that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk adjacent an air bearing surface (ABS) of the slider causing the slider to ride on an air bearing a slight distance from the surface of the rotating disk. When the slider rides on the air bearing the write and read heads are employed for writing magnetic impressions to and reading magnetic field signals from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
An exemplary high performance read head employs a spin valve sensor for sensing the magnetic field signals from the rotating magnetic disk. The sensor includes a nonmagnetic electrically conductive spacer layer sandwiched between a ferromagnetic pinned layer and a ferromagnetic free layer. An antiferromagnetic pinning layer interfaces the pinned layer for pinning the magnetic moment of the pinned layer 90xc2x0 to the air bearing surface (ABS). First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. A magnetic moment of the free layer is free to rotate upwardly and downwardly with respect to the ABS from a quiescent or zero bias point position in response to positive and negative magnetic signal fields from the rotating magnetic disk. The quiescent position of the magnetic moment of the free layer, which is preferably parallel to the ABS, is when the sense current is conducted through the sensor without magnetic field signals from the rotating magnetic disk. If the quiescent position of the magnetic moment is not parallel to the ABS the positive and negative responses of the free layer will not be equal which results in read signal asymmetry, which is discussed in more detail hereinbelow.
The sensitivity of the spin valve sensor is quantified as magnetoresistive coefficient dr/R where dr is the change in resistance of the spin valve sensor from minimum resistance (magnetic moments of free and pinned layers parallel) to maximum resistance (magnetic moments of the free and pinned layers antiparallel) and R is the resistance of the spin valve sensor at minimum resistance. Because of the high magnetoresistance of a spin valve sensor it is sometimes referred to as a giant magnetoresistive (GMR) sensor. Changes in resistance of the spin valve sensor are a function of cos xcex8, where xcex8 is the angle between the magnetic moments of the pinned and free layers. When a sense current is conducted through the spin valve sensor, resistance changes cause potential changes that are detected and processed as playback signals from the rotating magnetic disk.
The transfer curve for a spin valve sensor is defined by the aforementioned cos xcex8 where xcex8 is the angle between the directions of the magnetic moments of the free and pinned layers. The bias point should be located midway between the top and bottom of the transfer curve. When the bias point is located below the midway point the spin valve sensor is negatively biased and has positive asymmetry and when the bias point is above the midway point the spin valve sensor is positively biased and has negative asymmetry. The location of the transfer curve relative to the bias point is influenced by four major forces on the free layer of a spin valve sensor, namely a ferromagnetic coupling field HFC between the pinned layer and the free layer, a net demagnetizing (demag) field HD from the pinned layer, a sense current field HI from all conductive layers of the spin valve except the free layer, a net image current field HIM from the first and second shield layers. The strongest magnetic force on the free layer structure is the sense current field HI.
In the present invention a negative ferromagnetic coupling field xe2x88x92HFC is obtained for the purpose of counterbalancing other magnetic fields acting on the free layer so as to more adequately position the bias point on the transfer curve of the spin valve sensor. In a preferred embodiment this is accomplished by providing a pinning layer which is composed of platinum manganese (PtMn) and providing a first seed layer composed of nickel manganese oxide (NiMnO) and a second seed layer composed of tantalum (Ta) wherein the first seed layer interfaces the first read gap layer, which is composed of aluminum oxide (Al2O3), and the second seed layer is located between the first seed layer and the pinning layer. The invention further includes a copper (Cu) layer which is located between the free layer and a capping layer wherein the capping layer is preferably tantalum (Ta). The purpose of the copper (Cu) layer, which is also referred to as a spin filter layer, is to increase the magnetoresistive coefficient dr/R. Unfortunately, the spin filter layer reduces the magnitude of the negative ferromagnetic coupling field which is being sought for proper balancing of the free layer. Further, the spin filter layer can result in a decrease of the magnetoresistive coefficient dr/R instead of an increase.
The present invention obviates reduction of the negative ferromagnetic coupling field by oxidizing a top of the free layer before formation of the capping layer. This may be accomplished by first sputter depositing the top of the free layer, which may be nickel iron (NiFe) or cobalt iron (CoFe) or cobalt (Co), and then introducing oxygen into a sputtering chamber for oxidizing the top of the deposited layer. Accordingly, the free layer has an oxidized film portion and an unoxidized film portion wherein the oxidized film portion is located between the unoxidized film portion and the capping layer. In my experiments I have shown that without the spin filter layer the negative ferromagnetic coupling field xe2x88x92HFC is about xe2x88x9216 Oe, that when the spin filter layer is added the negative ferromagnetic coupling field xe2x88x92HFC is degraded to about xe2x88x928 Oe, and that when the top of a nickel iron (NiFe) free layer is oxidized before forming the capping layer that the negative ferromagnetic coupling field xe2x88x92HFC is restored to xe2x88x9216 Oe. Further studies optimized the magnetoresistive coefficient dr/R of the present invention by appropriately sizing the thickness of the copper layer. The magnetoresistive coefficient dr/R was maximized when the thickness of the copper layer was about 6 xc3x85. The invention also includes oxidizing fully or a top portion of the copper layer and/or oxidizing top portions of multiple films of the free layer and capping layers.
Another aspect of the invention is that when the copper spacer layer of the spin valve sensor is made thinner the dr/R is increased. However, when the thickness of the spacer layer is decreased the ferromagnetic coupling field increases which may adversely affect the biasing of the free layer. The present invention enables the spin filter layer to be employed for increasing the dr/R in combination with a thinner spacer layer for further increasing the dr/R. When a negative ferromagnetic coupling field xe2x88x92HFC of xe2x88x9216 Oe is obtained by the present invention the more positive ferromagnetic coupling field due to a thinner spacer layer is offset by the xe2x88x9216 Oe. A resultant xe2x88x928 Oe or lower can still be used effectively for properly biasing the free layer.
An object of the present invention is to provide a spin valve sensor wherein a negative ferromagnetic coupling field HFC is not degraded when a copper layer is employed between a free layer and a capping layer for the purpose of increasing the magnetoresistive coefficient dr/R of the spin valve sensor.
Another object is to accomplish the foregoing object as well as appropriately sizing the copper layer so as to optimize the magnetoresistive coefficient dr/R.
Other objects and attendant advantages of the invention will be appreciated upon reading the following description taken together with the accompanying drawings.